Problem:
Jack works in the hardware section of a department store. A customer comes in and buys 3 gallons of paint and 7 brushes, and pays $69.96, including 6% sales tax. Another customer buys 2 gallons of paint and 3 brushes and pays $42.40, including sales tax.
Required:
Question: Find the price of a gallon of paint and that of a brush.
Note: Provide support for rationale.